Development of a Water Purification Method by Injecting Surface Water to Hypolimnion in Dams and Reservoirs

在富营养化湖泊和水库中观察到的溶解氧（DO）耗竭会损害天然底栖生物并导致沉积物中营养盐的释放。针对这一问题，提出了将含有大量溶解氧的地表水注入溶解氧经常耗尽的底层的方法。在该系统中，注入到下层的地表水通过热交换器冷却到下层的水温，并在沉积物正上方形成薄的好氧层。因此，通过运行该系统可以抑制沉积物中营养盐的释放。通过该系统向低水层注入地表水对水质的影响已在小范围内得到证实。因此，在本研究中，我们通过在岩手县田濑湖（面积为6.0平方公里）进行现场实验，调查了在低水层中连续注入地表水的情况下水质恢复的程度。2001年的实验结果表明，在距实验装置至少20 m的范围内，浅水层溶解氧浓度得到了改善。2002年，对实验装置进行了改进，在距实验装置至少40 m的范围内，观测到了扩大的溶解氧浓度提高效应。在溶解氧浓度提高期间，底层PO_4^<3->-P浓度呈下降趋势，在系统安装点处最低。硝化过程中NH_4^+-N浓度降低。这些结果表明，地表水注入下层水体可使下层水体水质得到净化，即使在大面积水域如塔色湖，适量、连续注入地表水也可扩大其净水能力。

Dissolved oxygen (DO) depletion, observed in eutrophic lakes and reservoirs, damages natural benthic organisms and induces nutrient release from the sediment. For this problem, a method which injects surface water, including a large amount of DO into the bottom layer in which DO is often depleted, is proposed. In this system, surface water injected into hypolimnion is cooled down to a water temperature of hypolimnion by a heat exchanger, and forms a thin oxic layer immediately above the sediment. As a result, nutrients release from the sediment could be suppressed by operating this system. Effects of surface-water injection into hypolimnion on water quality by this system had been already proved in small-scale area. In this study, therefore, we investigated the degree of water-quality restoration in case of continuous surface-water injection in hypolimnion by field experiments in Lake Tase in Iwate Prefecture, which has 6.0km^2 surface area. Experimental results in 2001 showed that DO concentration in hypolimnion was improved within at least 20m from the experimental equipment. After that, some devices were improved, therefore, expanded effect on the enhancement of DO concentration in hypolimnion was observed within at least 40m from the experimental equipment in 2002. In the period when DO concentration had been improved, PO_4^<3->-P concentration in hypolimnion kept decreasing and it was lowest at the system-installed point. The nitrification was also showed, accompanied by decreasing of NH_4^+-N concentration. These results suggested that injection of surface water into the hypolimnion made water quality in the bottom layer purified, and that appropriate and continuous injection of surface water could expand its water-purification ability, even in a large-scale water area like Lake Tase.